This research aimed to study the wear amount and mechanism of a hydrodynamic journal bearing lubricated with magnetorheological (MR) fluid. The effects of bearing load, hard particle content in the MR fluid, magnetic field activation, and bearing sleeve material were experimentally investigated. Results revealed that the standard bronze sleeve experienced extreme wear with MR lubrication, two orders of magnitude higher than for oil lubrication, while the friction coefficient was almost 6 times higher, probably due to severe third-body abrasion. The least amount of wear among all tested materials was observed with a more flexible polymer sleeve, which showed ∼3.5 times more wear than the oil-lubricated bearing and a smaller increase in friction coefficient, around 2.6 times, as well as the formation of a possibly protective layer of crushed particles in the converging region of the film. The results suggest that polymers, and possibly also softer materials such as rubber, are a promising alternative for bearings lubricated with MR fluids under low-speed and high-load conditions.

The research presented in this dissertation shows experimentally and numerically how magnetorheological fluids could be used as lubricant in hydrodynamic journal bearings to improve film thickness at low shaft speeds, and friction at high shaft speeds.

Thin film lubrication problems frequently involve the use of lubricants with non-Newtonian characteristics, and a relatively simple viscosity model that can describe several non-Newtonian fluids is the Herschel–Bulkley relation. This relation can model solid-like properties of a lubricant at low shear stress using a yield stress, while at higher shear stress values shear-thinning or thickening can be included. In literature, this viscosity model has been combined with various governing equations to solve the non-Newtonian thin film problem, resulting in models that range from full 3D CFD simulations, to 1D Reynolds equation based methods. However, something that all of these approaches have in common is that they are either computationally expensive, can only be used for 1D geometries, or use non-exact, regularised versions of the Herschel–Bulkley model for reasons of numerical stability. This paper therefore introduces a method for solving a thin film problem with a non-regularised Herschel–Bulkley lubricant using the 2D generalised Reynolds equation, and this approach is shown to be fast without compromising on accuracy. The increased speed will allow the model to be used more efficiently in complex simulations or design optimisation scenarios.

Magnetorheological (MR) fluids are frequently reported to have potential as lubricants for hydrodynamic bearings operating at high loads, but no comprehensive effort has been made to investigate their performance under a variety of operating conditions. This paper, therefore, presents an extensive experimental and numerical investigation of an MR-lubricated hydrodynamic journal bearing subjected to different loads and magnetic fields, and compares these results to those of an oil-lubricated bearing. It is shown that by increasing the magnetic field strength, the performance characteristics of the bearing can be changed from low hydrodynamic friction and a high transition speed to high hydrodynamic friction and a low transition speed. Furthermore, it was found that the way in which these characteristics scale with increasing load differs for the MR- and oil-lubricated bearings. With MR lubrication, the relative change in characteristics with the application of a magnetic field is smaller at higher loads, due to the strong shear-thinning rheology of MR fluids. To include these effects in the model, a basic relation for the apparent MR viscosity as a function of shear rate, temperature, and magnetic field strength is introduced. Finally, the bearing was made from a polymer to improve wear resistance under MR lubrication, but a comparison with a Reynolds equation-based numerical model indicates possible performance degradation due to shape errors, which is a known issue with this bearing material.

A journal bearing test bench is used to find the transition speed between the hydrodynamic and mixed lubrication regimes for a modified magnetorheological (MR) fluid. It is shown that the transition speed of the bearing can be reduced by applying a local magnetic field near minimum film when it is lubricated with the MR fluid, and that this will only marginally increase friction. The lubricating performance of the MR fluid is compared to that of a reference oil, and all experimental results are compared with a Finite Element model based on the Reynolds equation.